19 August 2009
Posted in Center Information
Management of yard waste is a significant challenge in the US, where in 2005 13.1% of the 245 million tons of municipal solid waste was reported to be yard waste. Approximately 61.9% of yard waste is recycled (http://www.epa.gov/garbage/pubs/mswchar05.pdf). Because of its volume and potential use as a soil amendment, the disposal of yard waste in lined landfills is banned in most states. Consequently, yard waste is frequently composted or mulched. Composting involves the aerobic biological degradation of yard waste to a soil-like material, whereas mulching involves
size reduction and homogenizing without biological processes. Common uses of compost and mulch are on municipal parks, roadways, and horticultural purposes. Another common use of mulch is as landfill cover to avoid purchase of soil or alternative cover materials. This study investigates the use of mulch as daily and intermediate cover for landfills. Seven mulch samples were collected from different parts of Florida, and direct shear and hydraulic conductivity tests were conducted to evaluate their geotechnical and hydraulic properties. Mulch samples had internal friction angles in the range of 140-160 and had an interface friction angle with MSW in the range of 120-130. All mulch samples were cohesive and had some adhesive resistance against MSW. A slope stability analysis was carried out assuming both a linear plane of failure at the interface between the Municipal Solid Waste (MSW) and mulch used as daily and intermediate cover and a circular slip surface where mulch is used as daily and intermediate cover using the program Slope/W. Results for both intermediate and daily covers showed that all seven mulch covers used at a slope of 1:3 and 1:4 had factors of safety above 1.5; indicating a stable slope. All mulch samples had a hydraulic conductivity of 0.1 cm/sec with a porosity ranging from 0.56-0.64. The effects of temperature, moisture content and oxygen concentration on spontaneous ignition of mulch were investigated. Compacted shredded mulch (maximum dimension less than 2.5 cm) at various moisture contents and oxygen concentration were placed in a programmable oven and subjected to increasing temperature (30 C/min) to evaluate the auto-ignition temperature under different moisture contents and oxygen concentrations. The ignition temperature for mulch was found to be 2300 C; indicating the likelihood for spontaneous combustion in landfill to take place if mulch is applied as cover without mixing with soil.